The objective of these studies is to identify the molecular mechanisms responsible for neuron-induced AChR clustering, focusing on the role of the Rho GTPases. The proposed studies will utilize cultured muscle cell lines to measure the actions of Rho GTPase modulators and effectors on AChR clustering. Microinjection and transfection techniques, biochemical assays, mass spectrometry, and fluorescence imaging of living cells will be used to analyze the mechanisms by which the trans-synaptic signaling proteins agrin and laminin activate the Rho GTPases and trigger AChR redistribution on cell surfaces. These studies will provide important insights into the regulatory pathways that modulate the functional state of synapses in general. Since the regulation of receptor site density constitutes a mechanism for functionally altering neuronal circuits, these studies can yield information relating to the synaptic mechanisms underlying learning and memory. By defining mechanisms responsible for AChR clustering to ensure efficient cholinergic synaptic transmission, these studies will yield findings of relevance to the etiology of myasthenia gravis as well as to the cholinergic deficits found in Alzheimer's disease.